The first thing that writers should do is read. Reading is valuable for writers for three reasons: it improves your linguistic ability, it connects you with the community of other writers and gives you an idea of what they are thinking and it saves you the trouble of writing a book when you discover that someone else has already written it. Writers who don't read are probably engaged in an exercise of ego rather than a contribution to a cultural conversation. Reading is the primary way that writers acquire the tool of language. Read the kind of writing that you wish to write. Poets read poetry. Journalists read journalism. When you find the time (which you won't if you are serious about writing a book), read in other fields as well.
A large part of writing a book is simply doing it. Life is full of distractions and, frankly, things that are more fun to do than writing. Make a writing schedule and make yourself follow it. It doesn't matter what the schedule is, as long as it contains many hours. Some people are only productive in the early morning, while some can't get started until 10 at night. Be aware of when and under what circumstances you are most productive. Don't berate yourself when you don't follow the schedule; simply reorganize and start again. Include time in your schedule for walking or other physical exercise;, it will make a tremendous difference in the clarity of your mind.
Many beginning writers make the mistake of believing that if a 300-page book is good, a 600-page book would be better. Do not make this mistake. When you have finished your first draft, go through it like an invading barbarian and slay everything that doesn't earn its place. Readers are reading for themselves, not for you. Depending on the nature of the book, they are reading for enjoyment, for education or for information, but they are not reading for you. Keep the prose tight, interesting and relevant. Find everything in the draft that is self-indulgent and meandering, and cut it out. Keep it in a file of "excerpts" if it makes you feel better, but don't leave it in the draft.